Resolution

Go to the Messaging agent's Agent Settings Tab in GW Admin. Under Performance Preferences at the bottom of the page, see what Maximum number of users: is set to. It might be set to 100.

That value should default to 5120 and be able to set as high as 100,000. You can set to the max of 5000 (if more is required you can use the switch /maxusers=XXX in the startup files). Restart agent and that shall fix it.